The present invention relates to a flash memory device and a method for manufacturing a flash memory device, and more particularly, to a flash memory device having a low-k dielectric spacer and a method for manufacturing a flash memory device having a low-k dielectric spacer.
“Flash memory” is a special type of electrically erasable programmable read only memory (EEPROM) that is known in the art. A normal EEPROM only allows one location at a time to be erased or written, meaning that flash memory can operate at higher effective speeds when the system uses it to read and write to different locations at the same time. All types of flash memory and EEPROM wear out after a certain number of erase operations, due to wear on the insulating oxide layer around the charge storage mechanism used to store data. Flash memory is non-volatile, which means that it stores information on a silicon chip in a way that does not need power to maintain the information in the chip. In addition, flash memory offers fast read access times and solid-state shock resistance.
Flash memory typically stores information in an array of transistors, commonly referred to as “cells,” each of which traditionally stores one bit of information. Flash memory is based on the Floating-Gate Avalanche-Injection Metal Oxide Semiconductor (FAMOS) transistor which is essentially an n-type Metal Oxide Semiconductor (NMOS) transistor with an additional floating conductor “suspended” by insulating materials between the gate and source/drain terminals.
FIG. 1 is a cross sectional view of a conventional flash memory cell 500. The conventional floating gate flash memory cell 500 includes an n+ type source 504, a p type channel 505, an n+ type drain 512 and a p− type substrate 502. A floating gate 506 is sandwiched between an insulating dielectric layer 510 and thin tunnel oxide layer 514 over the channel 505. The floating gate 506 provides the memory storage element for the flash memory cell 500 and is electrically insulated from other elements of the memory cell 500 by the thin tunnel oxide layer 514 and the insulating dielectric layer 510. Control gate 508 is located on top of the insulating dielectric 510 and is positioned over the floating gate 506. The floating gate 506 is electrically isolated from the control gate 508 by the insulating layer 510 such as a layer of silicon dioxide (SiO2). The conventional flash memory cell 500 shown is basically an n-channel transistor with the addition of a floating gate 506. Electrical “access” or coupling to the floating gate 506 takes place only through a capacitor network of surrounding SiO2 layers and source 504, drain 512, channel 505, and control gate 508. Any charge present on the floating gate 506 is retained due to the inherent Si—SiO2 energy barrier height, thereby creating a non-volatile memory.
Typically, the structure of the conventional flash memory cell 500 includes a thin tunneling oxide layer 514 on the order of about 100 angstroms (Å), an abrupt drain junction, a graded source junction, oxide-nitride-oxide (ONO) inter-poly oxide and a short electrical channel length on the order of about 0.3 microns or micrometers (μm). Because the only electrical connection to the floating gate 506 is through capacitance, the flash memory cell 500 can be thought of as a linear “capacitor network” with an n-channel transistor attached thereto. The total capacitance of the cell 500 is approximately equal to the additive capacitance of the network. Coupling ratio terms for the flash memory cell 500, which are defined as the ratio of terminal voltage coupled to the floating gate, are typically defined as follows: control gate coupling ratio (GCR), drain coupling ratio (DCR) and source coupling ratio (SCR).
Programming a flash memory cell 500 means that charge (i.e., electrons) is added to the floating gate 506. A high drain to source bias voltage is applied, along with a high control gate voltage Vg. The control gate voltage Vg inverts the channel 505, while the drain bias accelerates electrons toward the drain 512. In the process of crossing the channel 505, some electrons will collide with the silicon lattice and become redirected toward the Si—SiO2 interface. With the aid of the field produced by the gate voltage Vg some of the electrons travel across the thin oxide layer 514 and become added to the floating gate 506. After programming is completed the electrons added to the floating gate 506 increase the cell's threshold voltage. Programming is a selectively performed on each individual cell 500 in an array of cells 500.
Reading a flash memory cell 500 is performed using a sense amplifier (not shown). For cells 500 that have been programmed, the turn-on voltage Vt of cells is increased by the increased charge on the floating gate 500. By applying a control gate voltage Vg and monitoring the drain current, differences between a cell with charge and a cell without charge on the respective floating gates can be determined. A sense amplifier compares cell drain current with that of a reference cell such as a flash memory cell 500 which is programmed to the reference level during a manufacturing test. An erased memory cell 500 has more cell current than the reference cell and therefore is a logical “1” whereas a programmed memory cell 500 draws less current than the reference cell and is a logical “0.”
Erasing a flash memory cell 500 means that electrons (charge) are removed from the floating gate 506. Erasing flash memory is performed by applying electrical voltages to many cells at once so that the cells 500 are erased in a “flash.” A typical erase operation in a flash memory cell 500 may be performed by applying a positive voltage to the source 504, a negative or a ground voltage to the control gate 508 and holding substrate 502 of the flash memory cells 500 at ground potential. The drain 512 is allowed to “float.” Under these conditions, a high electric field is present between the floating gate 506 and the source 504. The source junction experiences a gated-diode condition during erase and electrons that manage to tunnel through the first few angstroms of the SiO2 of the tunnel oxide layer 514 are then swept into the source 504. After the erase operation has been completed, electrons have been removed from the floating gate 506 thereby reducing the cell threshold voltage Vt. While programming is selective to each individual flash memory cell 500, an erase operation typically includes many flash memory cells 500 in an array being erased simultaneously.
As the cell sizes for flash memory 500 continue to be reduced, the capacitance measured between the gate 506 and the drain 512 increases thereby resulting in a reduction of GCR.
It is desirable to provide a flash memory cell that can be reduced in size relative to conventional flash memory cells while not reducing GCR and achieving good performance and reliability. It is desirable to provide to a flash memory device having a low-k (dielectric value) spacer and a method for manufacturing such a flash memory device having a low-k dielectric spacer.